The GFDL-CM4X Climate Model Hierarchy, Part I: Model Description and Thermal Properties

We present the GFDL-CM4X (Geophysical Fluid Dynamics Laboratory Climate Model version 4X) coupled climate model hierarchy. The primary application for CM4X is to investigate ocean and sea ice physics as part of a realistic coupled Earth climate model. CM4X utilizes an updated MOM6 (Modular Ocean Model version 6) ocean physics package relative to CM4.0, and there are two members of the hierarchy: one that uses a horizontal grid spacing of $0.25°$ (referred to as CM4X-p25) and the other that uses a $0.125°$ grid (CM4X-p125). CM4X also refines its atmospheric grid from the nominally 100 km (cubed sphere C96) of CM4.0–50 km (C192). Finally, CM4X simplifies the land model to allow for a more focused study of the role of ocean changes to global mean climate. CM4X-p125 reaches a global ocean area mean heat flux imbalance of $-0.02W{m}^{-2}$ within $O\left(150\right)$ years in a pre-industrial simulation, and retains that thermally equilibrated state over the subsequent centuries. This 1850 thermal equilibrium is characterized by roughly $400\text{ZJ}$ less ocean heat than present-day, which corresponds to estimates for anthropogenic ocean heat uptake between 1870 and present-day. CM4X-p25 approaches its thermal equilibrium only after more than 1000 years, at which time its ocean has roughly $1100\text{ZJ}$ more heat than its early 21st century ocean initial state. Furthermore, the root-mean-square sea surface temperature bias for historical simulations is roughly 20% smaller in CM4X-p125 relative to CM4X-p25 (and CM4.0). We offer the mesoscale dominance hypothesis for why CM4X-p125 shows such favorable thermal equilibration properties.